1. Field of the Invention
The invention relates to a digital memory circuit having a multiplicity of memory cells which can be addressed selectively in order to either write or read memory data from said memory cells. Memory circuits of this generic type are generally referred to by the acronym RAM (Random Access Memory) and are usually manufactured as integrated components on semiconductor chips. The field of the invention is what is referred to as “dynamic” RAM components (DRAMs) as are used as the main memory in computers. The subject matter of the invention is also a memory controller which can be used in conjunction with memory circuits of the above-mentioned generic type.
2. Description of the Related Art
To be able to communicate with other circuits, a RAM component has a plurality of groups of external terminals. These include the data terminals via which the memory data is input and output, and the address terminals at which the information for addressing the memory cells is applied. These also include a separate input with a plurality of terminals for receiving commands for initiating the various operations of the RAM component. In the case of RAM components whose memory cells are organized as a matrix composed of rows and columns and in which a read or write process is started at a row by activating a row selection line (word line) according to a row address, the following commands, inter alia, are customary: “Precharge” for setting all the column selection lines to a specific initial potential; “Activate” for setting a word line which has been addressed according to the row address to an activation potential, as a result of which all the memory cells of the respective row are opened for a possible access for reading or writing; “Read” for initiating a reading process at the memory cells in the activated row which have been selected by means of a column address; “Write” for initiating a writing process at the memory cells in the activated row which have been selected by means of a column address.
In addition to these commands, a multiplicity of other commands are also used in customary RAM components, for example a command “Select Component” (Chip Select) used to address the memory component as a whole, and a command “Set Mode Register” used to set specific operating parameters or operating properties of the component according to a specific setting information item. The setting information item is usually input via the address terminal, while said command is applied at the command input in order to store the setting information in a register. This information may, for example, predefine the length of the bursts of the memory data which is to be written in and read out and, if desired, predefine further parameters, for example specific waiting times within the sequence of specific memory operations. Further commands are “No Operation” for filling out waiting times, “Power Down” for a waiting state with reduced power consumption and, in the case of DRAMs, the commands “Autorefresh” used to bring about automatic refreshing of the data in the memory cells and “Selfrefresh” used to start up the periodic refreshing of the data in operating intervals of the component.
The commands mentioned above are only a few examples and not an exhaustive listing. The contents of the commands and also their diversity depend, of course, on the design of the memory component with the total number of different commands depending on the number of control operations. Since it is desired to keep the number of external terminals at a memory component as small as possible, it would be counterproductive to provide a separate terminal for each command. Separate terminals must, if necessary, be provided for superordinate commands such as, for example, the “Select Component” command, which are intended to maintain an operating feature over a sequence of other commands. These other commands are usually coded as m-bit digital words in parallel formats so that m parallel command terminals are sufficient to be able to distinguish between 2m different commands. A command decoder then decodes each received m-bit command code word, in parallel with the superordinate commands mentioned above, in order to condition an internal memory device for carrying out the operation required by the command.
The reception of the commands in parallel format permits a high working speed of the memory component. Any command can be received and decoded within a single clock period so that the rapid initiation of operations required by a command is possible. This is desirable or necessary for high speed commands such as, for example, “Precharge”, “Activate”, “Read”, “Write”, “Autorefresh” and some others. On the other hand, the number of possible commands in the prior art described above is dependent on the number of command terminals. To be more precise, the number of command possibilities is in a fixed relationship with the number of command terminals.
However, a digital RAM memory circuit is flexible in its possibilities of use the greater the variety of command possibilities. Therefore, what is needed is to increase the ratio of the number of command possibilities with respect to the number of command terminals in a RAM memory circuit.